Six days after 10,500 tons of crude oil were spilled in a collision off the country's west coast, weather officials warned the slick was threatening to reach shore on the nature reserve and tourist destination of Anmyeon Island, the Yonhap news agency reported.

Some 21,000 soldiers, police and volunteers have collected about 10 percent of the spill, aided by some 220 ships and 14 aircraft.

The Ministry of Maritime Affairs and Fisheries said four U.S. environmental experts are to visit the area to help with the recovery efforts and Singapore has agreed to lend a special aircraft to help beginning Friday, Korea.net reported.

Another problem is that at least 25 tons of absorbents are needed per day but there are only 5 tons left, officials said. Negotiations were under way to purchase more than 100 tons of absorbent from China and Japan and domestic manufacturers were ordered to step up production, Yonhap said.